The devices and methods described below relate to the fields of chemical mechanical polishing and control of slurry flow rates. The devices and methods may also be used in the grinding and polishing of wafers for the electronic materials and data storage industries.
Chemical mechanical polishing (CMP) is a process for very finely polishing surfaces under precisely controlled conditions. In applications such as polishing wafers and integrated circuits, the process is used to remove a few angstroms of material from an integrated circuit layer, removing a precise thickness from the surface and leaving a perfectly flat surface. The surface to be polished may be comprised of many materials, including various metals and silicates.
To perform chemical mechanical polishing, a slurry comprising a suitable abrasive, a chemical agent which enhances the abrasion process, and water is pumped onto a set of polishing pads. The polishing pads are rotated over the surface requiring polishing. The amount of polishing (the thickness removed and the flatness of the finished surface) is controlled by controlling the time spent polishing, the distribution of abrasives in the slurry, the amount of slurry pumped into the polishing pads, and the slurry composition (and other parameters). It is therefore important to control each of these parameters in order to get a predictable and reliable result from the polishing process. In particular, unreliable slurry flow rates cause fluctuations in removal rates and a large number of unacceptable finished wafers or circuits.
The slurry used for polishing is sensitive to degradation by the components in the slurry flow path. Whenever the slurry is subject to shear forces created by intrusive mechanical components such as pump impellers, pressure gauge taps, or flow meter vanes, its abrasive particles have tendency to agglomerate. This agglomeration results in uneven polishing, scratching, and other defects in the polished surface. Accordingly, peristaltic pumps are used to pump the slurry because these pumps have no impellers which impart shear forces to the slurry. However, flow rate is often measured with vaned flow meters or other intrusive and shear creating flow meters which rely of the insertion of physical structures into the slurry flow (any agglomeration is tolerated, and results in lower reliability and yield of the system).
The peristaltic pumps used in CMP systems typically perform with a linear or near linear relationship between the speed of the pump and the flow rates generated by the pump (the outlet pressure has little effect on pump output volume). This assumes that the pressure of slurry provided to the inlet of the pump is constant. When the inlet pressure varies, the speed of the pump required for a given flow rate changes. Fortunately, the pump speed proportionality constant (which relates flow rate to pump speed) varies linearly, or nearly linearly, with inlet pressure. The flow rate constant, and its relationship to inlet pressure, can be determined empirically for a polishing system. This constant can then be used to control the peristaltic pump to compensate for variations in slurry inlet pressure and provide more constant slurry flow rates to the polishing pads.
The pump speed proportionality constant M (in units of RPM/(ml/min) is derived from equations such as M=slope(inlet pressure)+c. The slope and constant c are derived empirically for a system by measuring the flow rate at various pump speeds for a variety of inlet pressures. The pump speed required to maintain a specified flow rate is governed by the equation RPM=Mxc3x97Flow rate. Thus, by sensing the inlet pressure of the slurry provided to the slurry pump, the pump speed required for a desired flow rate may be adjusted based upon the slurry inlet pressure (through application of a pump speed proportionality constant which is a function of inlet pressure), thereby isolating the system from slurry flow rate fluctuations caused by slurry inlet pressure fluctuations.
Chemical mechanical polishing systems are manufactured in a variety of configurations. For each system, the pump speed proportionality constant as a function of inlet pressure must be determined. This may be accomplished once for a line of CMP systems manufactured to the same specifications, or it may be done on every unit. To use the measured pump speed proportionality constant curve, the peristaltic pump inlet piping is fitted with an inlet pressure sensor and the pump motor is provided with an encoder to monitor pump speed. The pump controller is provided with a computer and software programmed to take input from the pressure sensor and the motor encoder, and receive operator input regarding the user""s desired slurry output flow rates and the proportionality constant curve. The computer is programmed to calculate the pump speed required to maintain the specified output flow rate given the sensed inputs, and to control the pump accordingly to maintain the desired output flow rate.